In the pic below, you'll see 4 openings that are cast into the front opening of our carbs, 3 at the bottom & 1 in the upper right quadrant.
During my various carb cleaning/refurbishing projects, I've figured out the purpose of each passage...... with the exception of the bottom right opening.
Looking at the underside of the carb, it appears to go all the way across the carb and possibly into the little brass tube located on the other side. Any attempts to blow air thru it left me with the impression that there was a blockage somewhere and all attempts to clear it were futile.
That wasn't going to work and I needed answers, so using an extra carb body that was already trashed out, I got my dremel tool out and went to work! Starting about in the middle of the carb, I began to grind off the top of the passage and head towards the brass tube. Finally just before reaching the tube, it ends with NO opening to any other passage. That's it... a dead-end passage of no use for our carbs.
So now when y'all clean your carbs, no need to worry about that passage being clogged! Happy days!